The exposure wavelength of the light sources of exposure apparatuses used to manufacture semiconductor devices is gradually becoming shorter in accommodation of increasingly higher levels of integration in the semiconductor industry. More specifically, exposure wavelengths are gradually becoming shorter in the manner of the g-line having a wavelength of 436 nm, i-line having a wavelength of 365 nm, KrF lasers having a wavelength of 248 nm and ArF lasers having a wavelength of 193 nm. Extreme ultraviolet (EUV) lithography has been proposed as an exposure technology that uses EUV light to enable the realization of finer pattern transfer. Here, EUV light refers to light in the wavelength band of the soft X-ray region or vacuum ultraviolet region. More specifically, EUV light refers to light having a wavelength of about 0.2 nm to 100 nm. Light having a wavelength in the vicinity of 13.5 nm can be used as EUV light.
Reflective masks are used in EUV lithography due to the small difference in absorptance with respect to EUV light between materials. Reflective masks have been proposed for use in EUV lithography that have a multilayer reflective film that reflects exposure light formed on a substrate, and a phase shift film that absorbs exposure light formed into a pattern on a protective film for protecting the multilayer reflective film. Light that has entered a reflective mask installed in an exposure apparatus is absorbed at the portion where the phase shift film pattern is present and is reflected by the multilayer reflective film at the portion where the phase shift film pattern is not present. As a result, a light image corresponding to the phase shift film pattern is transferred to a semiconductor substrate through the reflection optics. A portion of incident exposure light in the phase shift film pattern is reflected (phase-shifted) by a phase difference of about 180 degrees from light reflected by the multilayer reflective film. As a result, contrast is obtained between those portions where the phase shift film pattern is present and those portions where it is not.
Technologies relating to such reflective masks for EUV lithography and reflective mask blanks for the fabrication thereof have been disclosed in, for example, Patent Literature 1 to 3.
In addition, Patent Literature 4 discloses a phase shift mask blank having a light-semi-transmitting film on a transparent substrate, and the center line average roughness (defined in JIS B 0601 and indicated in nmRa) of the incident and emission surfaces of the light-semi-transmitting film with respect to exposure light is 0.1 to 50 nmRa.